1. Field of the Invention
This invention relates in general to photometric measuring devices and more specifically to a photometer which includes automatic calibration means based on logarithmic value measurement and comparisons.
2. Description of the Prior Art
There are various types of photometric systems associated with measuring the absorbance of a sample. These systems often incorporate manual means for the "zeroing" of the measuring circuit prior to measuring the desired sample. In conventional photometers, there are two adjustment knobs for this manual calibration. One knob is for adjusting the base line for displaying "zero" absorbance for a 100% transmittance signal. The other knob is to set the gain of the amplifier after logarithmic conversion to display a corresponding absorbance for a given transmittance. This is usually determined by inserting a filter or color solution of known absorbance value in the test location.
This conventional method is very cumbersome and it is often difficult to obtain and maintain a filter or solution with an accurately known absorbance value. This will often result in an inaccurate gain slope setting.
Scott, U.S. Pat. No. 3,579,105 discloses a digital measuring circuit for determining a mathematical relationship between a variable signal and a relatively constant "reference" signal. The circuit operates on the theory of supplying a blank input which is expected to be equal to the reference signal. The recalibration circuit causes a change in the amplitude value, as by attenuation, of the reference signal so as to create a balance between the blank signal and the reference signal. Actually, the circuitry of Scott provides, in digital form, the logarithm of the ratio of two electrical inputs and readjusts the function to equal zero when the two inputs are to be considered equal. The adjustment or attenuation factor is stored and then utilized as part of the computing mode to calibrate out any errors. This system still involves the concept of "zeroing" the equipment and thus, the circuitry necessary to accomplish this is extensive and somewhat complicated. Consequently, the Scott system has the disadvantage of being complex and costly. A further disadvantage is that the attentuation factor is a time-dependent variable which compounds the problem of establishing an accurate zero balance. Other U.S. patents showing related photometers and calibrating apparatus are: U.S. Pat. Nos. 2,909,724 to Onksen; 2,942,183 to Chance; 3,377,467 to Staunton et al; 3,535,637 to Goransson; 3,634,868 to Pelavin; 3,711,774 to Bohler; 3,743,429 to Kawai; 3,733,137 to Badessa and 3,818,198 to Walker et al.